1. Field of the Invention
This invention relates to gas chromatographic analysis equipments and concerns a method and a device to adjust the cooling temperature of a trap for samples to be analysed.
2. Description of the Prior Art
In equipments for gas chromatographic analysis so-called traps are used, which are placed along precolumns of gas chromatographic capillary columns, and whose main object is to perform the concentration of the sample to be analysed before it is forwarded along the column itself. Said traps are generally constituted by a metal jacket enclosing a section of pre-column or an initial section of gas chromatographic column. The concentration of the sample to be analysed is obtained by condensing the sample in said section of column or pre-column which is enclosed by the trap, through a cooling of the trap itself. The thus concentrated sample subsequently passes through the gas chromatographic column, when it is evaporated by heating the trap. Cooling is generally carried out by feeding a refrigerating fluid, preferably liquid nitrogen, into the hollow space existing between the column or the pre-column and the trap walls, while heating is generally obtained by Joule effect applying a potential difference at the two opposite ends of a resistor within the trap. Devices to feed nitrogen into the trap are known, comprising a vessel under pressure containing liquid nitrogen, a duct connecting the vessel with the trap and a solenoid valve opening and closing said duct.
Nitrogen feeding to the trap takes place thanks to the pressure existing within the vessel, the value of which is that of nitrogen vapor pressure.
In said known devices the adjustment of the trap cooling temperature can be performed exclusively by adjusting the opening time of the solenoid valve controlling the vessel/trap duct. In fact it is not possible to adjust the nitrogen flow to the trap with the necessary speed and precision by varying the pressure existing inside the vessel. For example a pressure increase could be obtained by heating the liquid nitrogen contained in the vessel or introducing a pressure gas, for instance helium, into the vessel itself. However both methods require too much time and are difficult to be controlled.
The adjustment of the nitrogen flow to the trap can be obtained also by varying the value of pressure losses along the vessel/trap duct, for instance using valves capable of supporting extremely low temperatures. Said valves however are very expensive and can be hardly used because ice tends to form on their electric contacts. As above said, the adjustment of nitrogen flow to the trap is generally obtained by varying the opening time of the solenoid valve controlling the vessel/trap duct. In order to make said regulation dependent only from the time, it is imperative to maintain the pressure inside the vessel at a constant value. For this reason the vessel is provided with a relief automatic device which restores the desired value of nitrogen vapour pressure when a temperature increase of liquid nitrogen, due to the unavoidable heat exchange between the vessel and the outside environment, causes a corresponding pressure increase.
Said regulation method however involves a first inconvenience due to the fact that between one opening of the valve and the following one it is necessary to wait that suitable balance conditions, under which the desired value of nitrogen vapour pressure is obtained, are restored within the vessel. This drawback strongly limits the use of said cooling devices in combination with so-called automatic samplers which automatically and in rapid succession perform the analysis of a large number of samples. Furthermore, each time that the vessel must be replaced for exhaustion of nitrogen, it is necessary to wait that in the new vessel the balance conditions are created again, which involves a suspension of analyses for a certain period of time. In order to reduce this drawback, large size vessels are generally used; however they are very expensive and cumbersome and impose heavy limitations to the allowed highest pressure value inside the vessel itself. Said limitation involves on its turn some inconvenience due to the fact that a low value of nitrogen vapour pressure inside the vessel makes it impossible to obtain a rapid cooling at very low temperatures of the trap. This is due particularly to the fact that, to have a regular flow of liquid nitrogen along the duct, it is necessary that the latter is brought to the temperature of liquid nitrogen itself, which requires some time.
On the other side, the difficulty of regulating the pressure of nitrogen fed to the trap makes it difficult to perform a precise regulation of the trap cooling temperature, mainly when relatively high (very near to 0.degree. C.) temperatures have to be reached. In fact, in the latter case, the nitrogen flow rate, as determined by the pressure existing inside the vessel, causes a too rapid cooling to allow a control and a suspension thereof with the necessary precision and at the right time by the temperature recording system and by the system actuating the solenoid valve controlling the vessel/trap duct.
To overcome said drawback, the temperature of liquid nitrogen could be increased by mixing it with another gas before feeding it into the trap. Said method however involves the need of using a gas absolutely devoid of water (to avoid ice formation) and preferably without oxygen which condenses at the temperature of liquid nitrogen. Moreover said method does not allow accurate regulation of the cooling temperature of the trap.
The known devices, therefore, besides being extremely expensive, involve a series of drawbacks which make the adjustment of the cooling temperature of the trap very complicated and rough, with subsequent negative effects on the reliability of the results obtained from the analyses performed with the gas chromatographic equipment.